What law did you use to solve this problem?

Imagine that you fill a balloon at sea level (1.0 atm) with 18 L of air. You are taking the balloon to Denver, Colorado, for a birthday party. Colorado is the “Mile High State,” and Denver has an air pressure of 0.85 atm. When you get to the party, what will the volume of the balloon be?

21 L; You can use Boyle's law.
0.047 L; You can use Boyle's law.
0.047 L; You can use Charles's law.
21 L; You can use Charles's law.